1. Field of the Invention
The present invention relates to an implantable stimulating device of the type having a morphology sensitive detector for detection of electrical signals originating from the heart. The invention also relates to a method for automatic creation of a template or reference signal for a morphology sensitive detector for cardiac signals.
2. Description of the Prior Art
The use of morphology sensitive detectors that compare heart signals to a template as a tool to determine the actual heart rhythm is known.
U.S. Pat. No. 5,797,399 discloses a pattern recognition system for use in an implantable cardioverter defibrillator for differentiating between normal and abnormal heart beats. A template standard is established that defines a median or other statistical measure of “central tendency” above which or below which actual sample values would be notable.
U.S. Pat. No. 5,280,792 discloses a method for automatically classifying intracardiac electrograms and a system for performing the method. The method employs neural networks which are trained through the use of supervised training with signals representative for arrhythmia and for normal sinus rhythm.
U.S. Pat. No. 5,240,009 discloses a medical device with morphology discrimination. The intracardiac electrogram is identified by determining, with respect to a waveform peak of the intracardiac electrogram, its amplitude, width and polarity. The identification criteria are averaged and stored to provide a standard complex. Subsequent complexes are compared to the stored standard complex. Such comparison includes comparing peaks of subsequent complexes with the peaks of a stored standard complex, aligning subsequent complexes with a stored standard complex, and providing a score associated with the comparisons and alignment.
U.S. Pat. No. 5,645,070 discloses a method of discriminating among cardiac rhythms of supraventricular and ventricular origin by exploiting the differences in their underlying dynamics reflected in the morphology of the waveform. A first cardiac rhythm electrogram of known origin is sensed and a phase space representation or trajectory is generated for use as a template. The template is used for comparison with the current waveform complexes. If the difference is sufficiently different then the rhythm is deemed to be of different origin compared to the template.
U.S. Pat. No. 4,905,708 discloses an apparatus for recognizing cardiac arrhythmias that digitizes analog signals which are obtained when carrying out sensing at the heart or on the body and carries out a first differentiation of the digitalized signals. The concept is to compare the first differential of the digitized electrogram of each heartbeat with what is established to be the first differential of the digitized electrogram for normal rhythm.
In a master thesis by Joakim Lingman at the Royal Institute of Technology, Department for Signals, Sensors, & Systems, an improved heartbeat detector is disclosed. A matched filter for heart beat detection is disclosed. In order to obtain a template for the matched filter, a prerecorded digitized IEGM of a certain length is divided into three parts. Around the sample value with the highest negative derivative a window is chosen. The convolution between these three possible QRS complexes and the total signal are then calculated. The complex generating the three largest values from the convolution is chosen as the template. Convolution may be described as a multiplication of all samples in two curves and then adding the products to a convolution sum. The convolution sum reaches a maximum if the two curves are identical.